1. Field of Invention
The present invention relates to a semiconductor device including an oxide semiconductor thin film layer primarily comprising zinc oxide with controlled orientations and a manufacturing method thereof.
2. Description of the Background Art
It has been known for many years that oxides such as zinc oxide or magnesium zinc oxide have excellent characteristics as a semiconductor (an active layer). In recent years, active research and development of a semiconductor thin film layer using these compounds have been made in order to apply such a semiconductor thin film layer to electronic devices such as a thin film transistor (hereinafter abbreviated as TFT), a light emitting device, and a transparent conductive film.
An oxide TFT including a semiconductor thin film layer made of zinc oxide or magnesium zinc oxide has greater electron mobility and better TFT characteristics than an amorphous silicon TFT having a semiconductor thin film layer of amorphous silicon (a-Si:H), which has been mainly used for liquid crystal displays. Another advantage of the oxide TFTs is that high electron mobility can be expected because a crystalline thin film is formed even at a temperature as low as a room temperature. These advantages have been encouraging the development of the oxide TFTs.
TFTs using an oxide semiconductor thin film layer, such as a bottom gate type TFT and a top gate type TFT, have been reported.
The bottom gate type TFT includes, for example, a substrate, a gate electrode formed on the substrate, a gate insulating film formed on the gate electrode, an oxide semiconductor thin film layer primarily comprising zinc oxide and disposed on the upper surface of the gate electrode, a pair of source/drain electrodes connected to the oxide semiconductor thin film layer. The bottom gate type oxide TFTs are manufactured in a similar process to bottom gate type amorphous silicon TFTs, a manufacturing process of which has been already industrialized. Such bottom-gate type configuration is widely used in zinc oxide TFTs.
On the other hand, the top gate type TFT includes, for example, a substrate, a pair of source/drain electrodes formed on the substrate, an oxide semiconductor thin film layer formed on the source/drain electrodes, a gate insulating film formed on the oxide semiconductor thin film layer, and a gate electrode formed on the gate insulating film.
In the bottom gate type TFTs, the oxide semiconductor thin film layer is formed on the gate insulating film. In the bottom gate type TFTs with such a structure, an insufficiently crystallized area of the oxide semiconductor thin film layer, which is deposited at an early stage of the oxide semiconductor thin film layer formation, inevitably functions as an active layer. This results in insufficient electron mobility. In the top gate type TFTs, on the other hand, the gate insulating film is provided on the oxide semiconductor thin film layer. This means the sufficiently crystallized area in the upper part of the oxide semiconductor thin film layer functions as an active layer. In this respect, the top gate type TFTs are more useful than the bottom gate type TFTs.
Conventionally, zinc oxide semiconductor thin film layers with high c-axis crystal orientation have been preferably used. In such zinc oxide semiconductor thin film layers, crystal grains have c-axes that are aligned in a direction perpendicular to the substrate. Use of a zinc oxide semiconductor thin film layer with preferred orientation in a certain direction is considered to increase the electron mobility of the thin film transistor (TFT) that includes the zinc oxide semiconductor thin film layer. It is known that a zinc-oxide-based film with high c-axis crystal orientation can be formed by means of sputtering at a temperature of 500 degrees C. or lower. However, no reports have been made about orientation control aiming at other orientations than c-axis orientation or about amorphous zinc oxide.
On the other hand, various research has been performed with respect to increasing c-axis orientation of the semiconductor thin film layer of TFTs. Also, a method of increasing the c-axis orientation of the pyroelectric part of thin film pyroelectric infrared detection elements is disclosed in Japanese Patent No. 2787198.
Zinc oxide with high c-axis orientation includes columnar structures extending along the film thickness direction, and many grain boundaries exist in the zinc oxide. In the grain boundaries, there exist lattice defects, crystal distortion, and dangling bonds. Therefore the oxide semiconductor thin film layer is thermally unstable. When the oxide semiconductor thin film layer undergoes heat treatment for certain purposes (e.g. formation of a gate insulating film), desorption of oxide and zinc occurs in the grain boundaries of the oxide semiconductor thin film layer. This results in defect levels that produce electrically shallow impurity levels and decrease the resistance of the oxide semiconductor thin film layer. If such an oxide semiconductor thin film layer is used as an active layer in a thin film transistor, the thin film transistor operates in a normally-on mode (depression mode). In other words, drain current occurs without applying gate voltage. In the depression mode operation, defect levels increase while threshold voltage decreases and the leak current increases. In addition, the grain boundaries serve as an energy barrier against electrons in the channel and decrease the electron mobility.
Such problems are more prominent in the top gate thin film transistors, in which the gate insulating film is deposited on the oxide semiconductor thin film layer, than in the bottom gate type thin film transistors.
In the oxide semiconductor thin film layer primarily comprising zinc oxide with high c-axis orientation that has columnar structures, etching occurs along the columnar structures. Such etching causes difficulties in microfabrication. The columnar structures increase roughness of the oxide semiconductor thin film layer surface and the increased roughness prevents the formation of a thin gate insulating film disposed thereon. It further causes breakdown of the gate insulating film due to an electric field concentration and increased leakage current of the gate insulating film.
As described above, the oxide semiconductor thin film layer with high c-axis orientation has defects in heat resistance, a microfabrication property (material property to facilitate microfabrication), and surface smoothness when used in thin film transistors as well as other semiconductor devices such as diodes and photoelectric conversion elements.